Add an @pure annotation

I'd like to have an annotation to declare an intent that a function will not have side effects. In the presence of this annotation, I'd like the compiler to warn me if the function has side effects.

In a lot of code, you have simple helper methods that are intended to be side-effect free. It's very easy for a side effect to be accidentally introduced at a later time. I'd like to have a warning issued for this so the developer can correct it.